Conventional alkaline electrochemical cells have an anode comprising zinc and a cathode comprising manganese dioxide. The cell is typically formed of a cylindrical casing. The casing is initially formed with an enlarged open end and opposing closed end. After the cell contents are supplied, an end cap with insulating plug is inserted into the open end. The cell is closed by crimping the casing edge over an edge of the insulating plug and radially compressing the casing around the insulating plug to provide a tight seal. A portion of the cell casing at the closed end forms the positive terminal.
Primary alkaline electrochemical cells typically include a zinc anode active material, an alkaline electrolyte, a manganese dioxide cathode active material, and an electrolyte permeable separator film, typically of cellulose or cellulosic and polyvinylalcohol fibers. The anode active material can include for example, zinc particles admixed with conventional gelling agents, such as sodium carboxymethyl cellulose or the sodium salt of an acrylic acid copolymer, and an electrolyte. The gelling agent serves to suspend the zinc particles and to maintain them in contact with one another. Typically, a conductive metal nail inserted into the anode active material serves as the anode current collector, which is electrically connected to the negative terminal end cap. The electrolyte can be an aqueous solution of an alkali metal hydroxide for example, potassium hydroxide, sodium hydroxide or lithium hydroxide. The cathode typically includes particulate manganese dioxide as the electrochemically active material admixed with an electrically conductive additive, typically graphite material, to enhance electrical conductivity. Optionally, small amount of polymeric binders, for example polyethylene binder and other additives, such as titanium-containing compounds can be added to the cathode.
The manganese dioxide used in the cathode is preferably electrolytic manganese dioxide (EMD) which is made by direct electrolysis of a bath of manganese sulfate and sulfuric acid. The EMD is desirable since it has a high density and high purity. The electrical conductivity of EMD is fairly low. An electrically conductive material is added to the cathode mixture to improve the electric conductivity between individual manganese dioxide particles. Such electrically conductive additive also improves electric conductivity between the manganese dioxide particles and the cell housing, which also serves as cathode current collector. Suitable electrically conductive additives can include, for example, conductive carbon powders, such as carbon blacks, including acetylene blacks, flaky crystalline natural graphite, flaky crystalline synthetic graphite, including expanded or exfoliated graphite. The resistivity of graphites such as flaky natural or expanded graphites can typically be between about 3×10−3 ohm-cm and 4×10−3 ohm-cm.
It is desirable for a primary alkaline battery to have a high discharge capacity (i.e., long service life). Since commercial cell sizes have been fixed, it is known that the useful service life of a cell can be enhanced by packing greater amounts of the electrode active materials into the cell. However, such approach has practical limitations such as, for example, if the electrode active material is packed too densely in the cell, the rates of electrochemical reactions during cell discharge can be reduced, in turn reducing service life. Other deleterious effects such as cell polarization can occur as well. Polarization limits performance and service life. Although the amount of zinc or other active material included in the anode can be increased by decreasing the amount of electrolyte, there are practical limits since utilization of the anode active material, will begin to decrease as more active material is packed into the anode in relation to the alkaline electrolyte. Similarly, the amount of manganese dioxide or other active material included in the cathode typically can be increased by decreasing the amount of non-electrochemically active materials such as polymeric binder or carbon conductive additive, a sufficient quantity of conductive additive must be maintained to ensure an adequate level of bulk conductivity in the cathode.
Other problems associated with improving alkaline cell capacity and performance is to reduce the rate of zinc corrosion in the anode. The presence of alkaline electrolyte, typically potassium hydroxide, in the zinc anode gradually leads to production of zinc oxide (ZnO) deposits directly on the zinc particles and within the anode core. This eventually placates the effectiveness of remaining zinc, thus reducing zinc utilization and reduces the cell's overall electrochemical efficiency. The accumulation of zinc oxide deposit can eventually shut down the cell. A byproduct of the corrosion reaction is production of hydrogen gas which gradually raises the cell's internal pressure.
Although amalgamation of zinc particles with mercury is known to retard the rate of zinc corrosion reaction and reduce the rate of hydrogen gas production, modern alkaline cell generally contain zero added mercury, because of environmental regulations. In such case the total amount of mercury in the cell is less than about 100 parts by weight mercury per million parts by weight zinc, typically less than about 50 parts by weight mercury per million parts by weight zinc. The addition of surfactants to the anode mixture or alloying the zinc particles with metals such as indium, bismuth or aluminum have been used to replace the beneficial effect of mercury.
Although such alkaline cells are in widespread commercial use there is still a need to retard the rate of zinc corrosion and simultaneously improve cell's discharge capacity and service life. The improved cell must also be cost effective and exhibit reliable performance as well as high capacity (mAmp-hours) for normal applications such as flashlight, radio, and audio players.